<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .color{
            background-color: red;
        }
    </style>
</head>
<body>
<ul id="box">
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
</ul>
<script src="./jquery.js"></script>
<script>
    $(function () {
        $("#box li").eq(3).addClass("color");
        console.log($("#box li").eq(3).siblings().length);
    })
</script>
</body>
</html>